I have noticed a git repository called metaswitch which has some of the FMJ
classes. I also note that some of the bugs I have found in FMJ have also
been fixed in the metaswitch repository.

Does anyone know anything about this and whether the FMJ code as modified by
metaswitch is worth a further look?

Metaswitch has (or at least used to have) a desktop application based on Jitsi Desktop. While branding and customizing it, they fixed various bugs in libjitsi and FMJ. Unfortunately, they rarely contributed these fixes back to Jitsi. Later, when BlueJimp became part of Atlassian and the libjitsi license changed from LGPL to Apache, they also stopped publishing the modified source to Github.

(Now before someone says this is incorrect: this is simply my public point of view.)

So yes, it might be worth looking at the differences. But given that FMJ development hasn't stopped, some Metaswitch branches are from 2013, and the last upstream import was from 2015, it might be quite some work to figure out what's worth to be backported.